First, view the image you wanted to display here, from your multiply or flickr.
Right click on the image, then click COPY IMAGE LOCATION

In our forum, post a topic.
In the topic body, click the "insert image icon" (with red circle) it will automatically insert an html tag called [ img ] [ /img ]
Right click with your mouse In between the [ img ] tags and click PASTE. It will paste our previously copied IMAGE LOCATION
Click post to finalize.

Hope this helps.
UPDATE:
The above instructions works for firefox browsers. For Internet Explorer, instead of clicking COPY IMAGE LOCATION when you right click, you must click on PROPERTIES.
A new window will appear, there's an "image location (URL)", highlight the value and copy it by pressing CTRL-C.
You can paste the copied location like the intruction above.